Bernstein waves in the Io plasma torus: A novel kind of electron temperature sensor

Abstract

During Ulysses passage through the Io plasma torus, along a basically north-to-south trajectory crossing the magnetic equator at R~7.8 R_J from Jupiter, the Unified Radio and Plasma Wave experiment observed weakly banded emissions with well-defined minima at gyroharmonics. These noise bands are interpreted as stable electrostatic fluctuations in Bernstein modes. The finite size of the antenna is shown to produce an apparent polarization depending on the wavelength, so that measuring the spin modulation as a function of frequency yields the gyroradius and thus the local cold electron temperature. This determination is not affected by a very small concentration of suprathermal electrons, is independent of any gain calibration, and does not require an independent magnetic field measurement. We find that the temperature increases with latitude, from ~1.3×10⁵ K near the magnetic (or centrifugal) equator, to approximately twice this value at +/-10° latitude (i.e., a distance of ~1.3 R_J from the magnetic equatorial plane). As a by-product, we also deduce the magnetic field strength with a few percent error.